1. Field of the Invention
The present invention concerns a telephone transmission circuit, and more particularly an improvement in a telephone transmission circuit of the type which incorporates an amplifier.
2. Description of the Prior Art
The use of amplifiers in telephone sets has recently received the attention of industry because of improved performance and reduced prices of such sets resulting from integrated circuit development.
This type of transmission circuit proposed heretofore in the art comprises at its two line terminals, a series circuit including two resistors, a power circuit for a receiver amplifier and a balancing network (See "Electronic Push-button Telephone Set - ERICOFON 700" by Arne Boeryd et al, ERICSSON REVIEW, No. 3, 1976, pp. 118-132). Between the connecting point of two resistances and one of the line terminals are connected a power circuit for the transmitter amplifier and its output circuit. The input for the receiver amplifier is directly connected to the other of the line terminals, while a power circuit for the receiver amplifier is connected between the one line terminal via the balancing network and the end of a series connection of the two resistances opposite to the other line terminal. Thus, the power circuit for the receiver amplifier is supplied electric current from the line terminal via the two resistances resulting in power loss. As for the transmitter amplifier, a power source current and an output signal current are supplied via one resistance, thereby unavoidably lowering the maximum distortionless output. With the conventional type transmission circuit, the impedance of the balancing network must be small, resulting in an increase of the size of the capacitor used. As the impedance of the transmission circuit depends on the impedance of the balancing network, a change of the impedance of the balancing network adversely affects the desired matching of the line impedance and the transmission circuit impedance.